Circuit and method for coil current control

ABSTRACT

Circuits and methods to control a current in a coil are disclosed. The circuit and methods provide over-dwell protection and soft shut-down functionality to safely discharge the coil. The safe discharge of the coil is facilitated by a soft-start ramp signal that reduces the coil current gradually by controlling a switching device according. A profile of the soft-start ramp signal over time determines the gradual reduction. The profile of the soft-start ramp signal can be adjusted to set (i) an over-dwell period of the coil current, after which the coil current is shut down, and (ii) a soft shut-down period, over which the coil current is gradually reduced.

FIELD OF THE DISCLOSURE

The present disclosure relates to electronic systems thatcharge/discharge a coil and more specifically to circuits and methodsfor controlling a current in the coil.

BACKGROUND

Systems that utilize a coil typically include circuits to protect thesystem from damage that could arise as the coil is charged anddischarged. For example, overheating could result if a coil is chargedand operated at a high current over a period that is too long. Inanother example, a kickback (i.e., flyback) voltage (e.g., voltagespike) could lead to an unwanted voltage breakdown (e.g., spark) if acoil is discharged over a period that is too short. Accordingly, newcircuits and methods are needed to control a current in a coil.

SUMMARY

The present disclosure describes circuits and methods to control acurrent in a coil. The coil may be used as part of an ignition systemand the circuit and method can provide over-dwell protection and softshut-down functionality to safely discharge the coil without producingan unintentional spark. The circuits and methods generate a soft-startramp signal that can adjust the coil current by controlling a switchingdevice (e.g., insulated gate bipolar transistor) according to a profile(i.e., shape) of the soft-start ramp signal over time. The profile ofthe soft-start ramp signal can be adjusted to set an over-dwell periodof the coil current, after which the coil current is shut down. Theprofile of the soft-start ramp signal can also be adjusted to set a softshut-down period, over which the coil current is gradually reduced(e.g., to zero amperes).

In one aspect, the present disclosure describes a circuit forcontrolling a current in a coil. The circuit includes a capacitorconnected between a source voltage (Vs) and an output node. The circuitalso includes a first switching device that is connected between thevoltage source and the output node. When the first switching device isin an ON state, the output node is coupled to the voltage source, andwhen the first switching device is in an OFF state, the output node iscoupled to the voltage source through the capacitor. The circuit furtherincludes a voltage-controlled current-source (VCCS) that is connectedbetween the output node and a ground, the VCCS outputs a current thatcharges the capacitor so that when the first switching device is movedfrom the ON state to the OFF state, a voltage is generated at the outputnode that decreases over a period from Vs to a ground voltage accordingto a soft-start-profile.

In another aspect, the present disclosure describes a method forcontrolling a current in a coil that includes receiving a charge coiltrigger signal, generating a soft-start ramp signal that has a voltageprofile that decreases with time, and applying the soft-start rampsignal to a gate of a switching device to gradually shut down thecurrent in the coil. The current in the coil is gradually shut down overa soft shut-down period and after an over-dwell period. Each periodcorresponds to the voltage profile of the soft-start ramp signal.

In a possible embodiment of the method, the step of generating asoft-start ramp signal includes coupling a first side of the capacitorto a source voltage and controlling a current charging the capacitorusing a voltage controlled current source (VCCS). The VCCS is connectedbetween a second side of the capacitor and a ground voltage and avoltage across the capacitor can be applied to the input of the VCCS tocontrol the current charging the capacitor. The soft-start ramp signalis then output as the voltage at the second side of the capacitor.

In another aspect, the present disclosure describes a soft-start rampgenerator. The soft-start ramp generator includes voltage sourceterminal, a ground terminal, and an output node. A capacitor is coupledbetween the voltage source terminal and the output node, and a voltagecontrolled current source (VCCS) is coupled between the output node andthe ground terminal. The VCCS is configured to control the currentthrough the capacitor based on the voltage across the capacitor so thata voltage at the output node deceases from a voltage at the voltagesource terminal to a voltage at the ground terminal according to asoft-start profile.

In a possible embodiment of the soft-start ramp generator, the voltageat the output node of the soft-start ramp generator can be coupled to aswitching device to control a current in a coil. The current in the coilis controlled to gradually decrease over a soft shut-down period afteran over-dwell period.

The foregoing illustrative summary, as well as other exemplaryobjectives and/or advantages of the disclosure, and the manner in whichthe same are accomplished, are further explained within the followingdetailed description and its accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an ignition system according to a possibleembodiment of the present disclosure.

FIG. 2 schematically depicts signals in an ignition system.

FIG. 3 is a block diagram that schematically depicts a system forcontrolling the current in a coil.

FIG. 4 shows three related graphs that illustrate the operation ofcontrolling coil current.

FIG. 5 is a graph of a soft-start ramp signal.

FIG. 6 schematically depicts a soft-start ramp generator circuit.

FIG. 7 is a graph illustrating the operating characteristics of avoltage controlled current source (VCCS).

FIG. 8 schematically depicts circuit for controlling a current in acoil.

FIG. 9 is a flow chart of a method for controlling a current in a coil.

The components in the drawings are not necessarily to scale relative toeach other. Like reference numerals designate corresponding partsthroughout the several views.

DETAILED DESCRIPTION

Various systems may use a coil to store and supply energy. An example ofsuch a system is shown in FIG. 1. Specifically, FIG. 1 illustrates asimplified block diagram of an ignition system 100 for an engine (e.g.,in a vehicle). The ignition system produces a spark at a spark gap 135in response to a control signal. In operation, a first signal (e.g., arising edge of a pulse) is sent from an engine control unit (ECU) 110 toa current controller 115. In response to the first signal, the currentcontroller 115 applies a signal to a switching device 140 to control itsconduction to charge a coil 120. Specifically, the switching device 140is configured to conduct so that a coil current to flows from voltagesource (i.e., VBAT) 130 to a ground voltage 145 through the coil 120,After the coil 120 is charged, the ECU can send a second signal (e.g., afalling edge of a pulse) to the current controller 115. Based on thissecond signal the current controller 115 applies a signal to theswitching device 140 to control its conduction to discharge the coil120. Specifically, the switching device 140 is configured to ceaseconducting a coil current. The abrupt cessation of coil current producesa large kickback voltage across the coil 120. This voltage can be madeeven larger by inductively coupling the coil 120 to a second coil 125(e.g., in a step-up transformer configuration). The voltage induced inthe second coil 125 is coupled to a spark gap 135 and is large enough toovercome the breakdown voltage of the spark gap to produce a spark.

In certain circumstances, the first signal (i.e., ON signal) from theECU 110 is either not followed by the second signal (OFF signal) orfollowed much later by the second signal. As a result, the coil currentis allowed to charge to a large value without producing a spark. If thecoil current is not shut down within some reasonable time period, thehigh coil current could cause damage to the ignition system (e.g., byoverheating). Accordingly, to prevent damage, the coil can be dischargedafter a pre-determined over-dwell period (ODP).

To discharge the coil 120 the switching device 140 may be configured toabruptly stop conducting (i.e., hard shut down), but as describedpreviously, a hard shut down can cause a spark at the spark gap 135.Because a discharge after an ODP is generally not correlated with normaloperation, a spark produced by a hard shut down may be undesirable. Inthe example of an ignition system, a spark resulting from a hard shutdown after an ODP could result in damage to and/or poor operation of theengine because it may not be correlated in time with the appropriatecycle (i.e., stroke) of the engine. In order to prevent the untimelyspark, the switching device may be configured to discharge the coil 120gradually (i.e., soft shut down) by reducing the coil current over asoft shut down (SSD) period.

The gradual discharge may be characterized as a current reduction overtime that results induced coil voltage below a particular value. Inother words, the change in current per unit time during a gradualdischarge of a coil can be kept below a maximum value to prevent aninduced voltage across the coil (i.e., coil having an inductance, L)from exceeding a maximum value. In particular, a gradual discharge(i.e., gradual shut down, soft shut down) of a coil may refer to a coilcurrent profile that conforms to:

dI/dt<V _(MAX) /L

In an ignition system, for example, a gradual reduction of coil current(i.e., soft shut down) may be at a rate that is at or below a maximumcurrent reduction rate of about 2.5 amperes per millisecond (A/ms). Insome embodiments, the maximum current reduction rate depends on theinductance of a primary coil and a turns ratio (i.e., between a primaryand a secondary coil). For these embodiments, a maximum currentreduction rate may in the range of 1 A/ms to 2.5 A/ms.

FIG. 2 illustrates two possible ECU signal scenarios 210, 220 that causea coil current, I_(C), 230 to exceed an ODP 236. In both cases, at time,t1, a first trigger 212 (e.g., rising edge of a pulse) is received tostart a charging a coil. During charging, the coil current 230 risesduring a (brief) transient period 235 from time, t1, to time, t2 untilit reaches a maximum current 239 set by the parameters of the circuit(e.g., coil resistance, switching device resistance, battery voltage,etc.). In the first ECU signal scenario 210, a second trigger todischarge the coil is never provided, while in the second ECU signalscenario 212, a second trigger 224 (e.g., falling edge of pulse) isprovided at time, t5, which is much later than the ODP 236. In either ofthe ECU signal scenarios 210, 220, the coil current starts normally attime, t1 but is never triggered to stop flowing. The lack of a controlsignal could result in the coil current, I_(C), remaining at its maximumvalue 239 for a time that exceeds a predetermined ODP 236 (e.g.,predetermined as safe). Accordingly, a current controller may includecircuits and/or devices that handle these scenarios by automaticallyconfiguring the switching device to reduce the coil current after theODP 236. The coil current is reduced gradually according to a soft shutdown period 238.

The two possible ECU signal scenarios are presented as examples, and thepresent disclosure is not limited to these particular examples. Rather,the present disclosure embraces all possible scenarios that require acoil current to be shut down gradually after some predetermined period.Accordingly, an aspect of the present disclosure is a circuit thatcontrols a coil current to automatically turn off gradually over a softshut down period after an over-dwell period has expired. In particular,the circuit disclosed provides independent control (i.e., adjustment) ofthe over-dwell period and of the soft shut down period. Further, thisadjustment may occur in real time (e.g., based on a signal correspondingto an engine condition) or may be set once (e.g., factory set).

FIG. 3 includes is a block diagram of a system for controlling thecurrent in a coil according to a possible embodiment of the presentdisclosure. The system includes a coil 350 and an insulated gate bipolartransistor 360 connected in series. The coil current, I_(C), 340 may beadjusted by applying a signal to a terminal of the IGBT. For example, agate (G) to emitter (E) voltage, V_(GE), may control a collector (C)to-emitter (E) current, I_(CE), according to a current versus voltage(I-V) characteristic of the IGBT (e.g., see FIG. 4, 415).

The system uses a coil-current control circuit (i.e., current controlcircuit, current controller, controller, etc.) 300 to apply a voltage tothe gate of the IGBT in response to a signal from the ECU 320. Forexample, if the ECU signal 320 transitions from a relatively low voltage(i.e., logic low) to a relatively high voltage (i.e., logic high), thenthe current controller can provide a signal to drive (e.g., charge) thegate of the IGBT so that the IGBT transitions from an OFF state(I_(C)=0) to an ON state (I_(C)>0). Continuing the example, if the ECUsignal 320 subsequently transitions from the logic high to the logiclow, then the current controller 300 can drive (e.g., discharge) thegate (G) of the IGBT 360 so that the IGBT transitions from the ON stateto the OFF state.

To prevent the IGBT 360 from remaining in the ON state long enough tocause damage, the control circuit 300 includes a soft-start rampgenerator 310. The soft-start ramp generator produces a signal that whencoupled to the gate of the IGBT allows the coil current to remain ON foran over-dwell period and then to shut OFF gradually. In other words, thevoltage profile of the soft-start ramp signal (i.e., wave), whencombined with the particular I-V characteristic of the IGBT, controlsthe coil current according to the IC profile 230 shown in FIG. 2.

The correspondence between a soft-start ramp signal 410, an I-Vcharacteristic of the IGBT 415, and the coil current 420 is shown inFIG. 4. Referring to the coil current graph 420, at time, t0, an triggersignal causes the current controller 300 to turn the IGBT ON and thecoil current 420 quickly rises to a maximum current (I_(coilmax)) 440.Referring to the ramp generator output graph 410, at the same time (t0)the soft-start ramp generator is started. The graphs are related by thegraph 415 of collector-emitter current (ICE) versus gate-emitter voltage(VGE) that characterizes the IGBT (i.e., the I-V characteristic of theIGBT). The ramp generator output voltage corresponds to V_(GE), theV_(GE) corresponds to I_(CE), and the I_(CE) corresponds to I_(C).

The soft-start ramp signal shown in the ramp generator output graph 410derives its name from the period after t0 when the voltage nonlinearlydeclines from its initial value (i.e., the ramp voltage is slow to startits downward progression towards zero volts). The soft-start ramp signalis applied to the gate of an IGBT to produce a gate-emitter voltage,V_(GE) that corresponds to a collector-emitter current (I_(CE))according to the IGBT I-V characteristic current 415. Thecollector-emitter current corresponds to the coil current 420 exceptwhen limited to I_(coilmax) 440 by operating characteristics of thedevices in the system (e.g., the resistance of the IGBT and/or the coil,battery voltage). Without these limitations, the coil currentcorresponding to the ramp signal would follow the dotted portion 445 ofthe coil current 420 profile.

To better understand the correspondence of the graphs shown in FIG. 4,the coil current for a selected time may be determined as an example.Starting at the ramp generator output graph 410, a time, t4, isselected. At the selected time (t4) the ramp generator output voltage425 has fallen from its initial value. The ramp voltage 425 at t4 isapplied to the gate of the IGBT and thus corresponds to a current 430(i.e., I_(CE) current) in the IGBT, which in turn, corresponds to acurrent (i.e., I_(C)) of the coil 435 at the selected time, t4.

A profile of the coil current is generated over time based on (i) theprofile (i.e., shape) of the soft-start ramp generator signal (i.e.,wave), (ii) the I-V characteristic profile of the IGBT, and (iii) themaximum coil current 440 supported by the IGBT/coil/battery voltage. Thecoil current profile includes an over dwell portion 450 and a soft shutdown portion 455. For the example shown in FIG. 4. The over-dwell periodcan be controlled by how fast the ramp generator output voltagedecreases in a first portion 460 (i.e., how V_(GE) decreases I_(CE) to avalue corresponding to the maximum current in the coil, I_(coilmax)).Likewise, the soft shut down period (and shape) can by determined by theshape of the soft-start ramp generator signal 410 in a second portion465 (i.e., how quickly the V_(GE) is decreased to the threshold voltageof the IGBT).

To achieve independent control over the over-dwell period and the softshut down period, a soft-start ramp signal may be generated that can bedivided into three main portions. An example of a soft-start ramp signal500 is illustrated in FIG. 5. After the soft-start ramp signal isstarted at t1, the ramp remains substantially at its maximum value,V_(SOURCE). In practice there may be a very small decrease in voltageduring the substantially constant portion 510. This portion can allowthe transients to settle and prevent sudden changes in the circuitbefore voltages begin to monotonically decrease. The substantiallyconstant portion 510 typically has a relatively short duration comparedto the overall duration of the ramp signal (e.g., <5%), but in someembodiments, the substantially constant portion 510 may be adjustable toextend the over-dwell period. After the substantially constant portion510, the ramp signal begins a soft-start portion 520. During thesoft-start portion the ramp signal decreases nonlinearly. The rate ofdecrease of the ramp signal at time, t2, is relatively slow compared tothe rate of decrease of the ramp signal at time, t3. The change in therate of decrease may be adjusted to control the over-dwell period. Afterthe soft-start portion 520, a linear portion 530 begins at t3 andcontinues until the voltage reaches zero at t4. The time at which thelinear portion begins (i.e., t3) and/or the slope of the linear portionmay affect the soft shut down period. In some implementations, thelinear portion begins sometime after the over-dwell period and duringthe soft shut down period of the coil current profile.

The linear portion ramp signal may result from limitations of thecircuit generating the ramp signal. For example, the growing rate ofvoltage decrease of the soft start period may be limited to some maximumvalue. In this embodiment, the linear portion 530 begins when this limitis reached. In other embodiments, the linear portion 530 is not present,and the soft-start portion 520 extends until the voltage reaches zero.This is especially true for embodiments in which the substantiallyconstant portion 510 is extended in time.

FIG. 6 depicts an example of soft-start ramp generator circuit 600 thatis configured to generate a ramp signal that is the same, or similar, toramp signal shown in FIG. 5. The circuit 600 includes a capacitor 620that is coupled between a voltage source 610 (V_(SOURCE)) and a voltagecontrolled current source (VCCS) 640. The VCCS 640 is coupled between alow a ground voltage 650 (e.g., zero volts) and an output 630 of thecircuit 600 where the capacitor 620 and the VCCS 620 are coupledtogether.

In operation, the capacitor is charged by the voltage source 610according to a current (I_(VCCS)) 670 of the VCCS 640. During thecharging, I_(VCCS) is based on the voltage across the capacitor, V_(CAP)660. Thus, as the capacitor 620 is charged (i.e., as V_(CAP) increases),the rate at which the capacitor is charged also increases (i.e.,I_(VCCS) increases). This configuration leads to a nonlinear increase ofthe voltage across the capacitor. Because the voltage at the output 630is the source voltage (V_(SOURCE)) minus the voltage of the capacitor(V_(CAP)), the output voltage (V_(RAMP)) has the nonlinearly decreasingprofile according to the soft-start ramp portion 520 of FIG. 5.

In addition to the soft-start portion 520, the ramp signal at the output630 may also include a substantially constant portion 510 and a linearportion 530 (see FIG. 5). Accordingly, VCCS may have different operatingcharacteristics that depend on the voltage of the capacitor 660. A graphillustrating possible operating characteristics of the VCCS 640 is shownin FIG. 7.

When the soft-start ramp generator circuit is activated (e.g., receivesa trigger signal at t0), a small current I_(START) 701 flows to startcharging the capacitor. In a first operating characteristic portion 710,the current of the VCCS is constant until the voltage of capacitorreaches a voltage, V_(BIAS) 705. The time it takes for the capacitor tocharge to this voltage depends on I_(START) and V_(BIAS), which can beadjusted in the VCCS. For example, V_(BIAS) can be made relatively smallso that the voltage across the capacitor does not significantly changeduring the period over which the capacitor is charged to V_(BIAS).Accordingly, this period may correspond to the substantially constantportion 510 of the soft-start ramp signal (V_(RAMP)) shown in FIG. 5.

After the voltage of the capacitor, V_(CAP), is charged above V_(BIAS)705, the VCCS exhibits a second operating characteristic portion 725 inwhich the output current (I_(VCCS)) depends (e.g., linearly) on theinput voltage (V_(CAP)). For voltages in the range from V_(BIAS) 705 toV_(I) 730, I_(VCCS) changes linearly from I_(START) 701 to its maximumvalue I_(CHG) 720. As I_(VCCS) increases, V_(CAP) increases, which inturn, increases I_(VCCS). Thus, the capacitor is charged nonlinearlyover time because a positive feedback loop is formed and the result ofthe positive feedback loop (i.e., V_(CAP)) grows nonlinearly.

The ramp signal (V_(RAMP)) at the output 630 of the ramp generatorcircuit 600 is V_(SOURCE)-V_(CAP). Thus, as V_(CAP) grows nonlinearlyover time (i.e., until it reaches V_(SOURCE)), V_(RAMP) decreasesnonlinearly over time until it reaches to zero. The rates of voltagedecrease (i.e., shape) of the ramp signal depends, at least, on theslope, dI/dV, 715 for the range of voltages between V_(BIAS) 705 andV_(I) 730. The period during which the voltages decrease over this rangecorresponds to the soft-start portion 520 of the ramp signal (V_(RAMP)),as shown in FIG. 5.

In some embodiments, the maximum current provided by VCCS is reached ata voltage, V_(I) 730, which is less than the fully charged capacitorvoltage of V_(SOURCE) 740 (e.g., 55% of V_(SOURCE)). This creates athird operating characteristic portion 735 of the VCCS. For voltages(V_(CAP)) in the range of V_(I) to V_(SOURCE), the current provided bythe VCCS is constant. During this mode of operation, the capacitor ischarge linearly with time. Accordingly the voltage at the output of theramp circuit declines linearly with time. Thus, the period during whichthe voltages at the output 630 decrease from V_(I) to V_(SOURCE)correspond to the linear portion 530 of the soft-start ramp signal(V_(RAMP)), as shown in FIG. 5.

A final operating characteristic of the VCCS occurs when the capacitoris fully charged (i.e., V_(CAP)=V_(SOURCE)). When the capacitor becomesfully charged, the output 630 of the ramp generator circuit reaches zerovoltage (i.e., V_(RAMP)=0). Additionally, the voltage across the VCCS iszero. Thus, when V_(CAP) reaches V_(SOURCE) 740, the current I_(VCCS)drops to zero 740 because VCCS cannot feed an output current in thiscondition (i.e., because the driver transistor of the current sourcecannot operate).

The shape of the ramp signal is thus adjustable by configuring thecharacteristics of the VCCS. For example, one or more of the parametersI_(START) 701, I_(CHG) 720, V_(BIAS) 705, and V_(I) 730, V_(SOURCE) 740may be controlled to adjust the shape of the soft-start ramp signal.Because the shape of the soft-start ramp signal contributes to theprofile of the coil current, a particular set of values for theseparameters may be derived to generate a coil current profile having aparticular over-dwell period and a particular soft shut down period.

Thus, an aspect of the disclosed circuits and methods is a rampgenerator circuit with a ramp signal output that is based on a voltageacross a capacitor charged by a VCCS. Another aspect of the disclosedcircuits and methods is an output of the VCCS is controlled by thevoltage across the capacitor with respect to an operating characteristicprofile of the VCCS, and a portion of the operating characteristicprofile linearly increases the current charging the capacitor based onthe voltage across the capacitor.

A possible embodiment of a current control circuit 800 that includes thesoft-start ramp generator circuit 801 as described previous is shown inFIG. 8. The soft-start ramp generator circuit 801 includes a capacitor812 in series with a VCCS 814. The current of the VCCS 814 charges thecapacitor to a source voltage 810 when a trigger signal (e.g., a risingedge of pulse transitioning from a lower voltage level to a highervoltage level) is received at an input 802. Additionally, a voltageacross the capacitor 812 and the operating characteristics of the VCCS814 (i.e., at the voltage across the capacitor) determine the currentoutput of VCCS.

The current control circuit 800 further includes a non-invertingamplifier 816. The ramp signal at the output 803 of the ramp generatorcircuit 801 (i.e., the node between the capacitor 812 and the VCCS 814)is fed to the non-inverting amplifier 816. The non-inverting amplifier816 may be embodied in various ways including (but not limited to) anoperational amplifier configured as a buffer amplifier (e.g., with unitygain). The non-inverting amplifier 816 functions to prevent loadingissues on the ramp generator circuit and to provide sufficient currentto drive a gate of a switching device.

The switching device of the current control circuit 800 may be anyswitching device for which a current between a first and a secondterminal can be controlled by a signal at a third terminal. An IGBT 824is used in the embodiment shown in FIG. 8. The IGBT 824 is connected atits gate to the output of the non-inverting amplifier 816, at itsemitter to a ground voltage, and at its collector to a coil 826.Accordingly, the ramp signal at the output of the non-invertingamplifier can control V_(GE) of the IGBT to adjust the current flowingthrough the coil (i.e., I_(CE) of the IGBT).

The current control circuit 800 further includes a network of switchingdevices and gate control electronics to control the start of the rampgenerator circuit and to control the charging and discharging of thecoil 826. A differential amplifier 804 may be used at the input 802 ofthe current control circuit 800 to compare an input voltage to areference voltage 806. Based on this comparison, the differentialamplifier 804 can output a lower voltage or a higher voltage to controlthe network of switching devices.

The network of switching devices may include a ramp-circuit switchingdevice 808. The ramp-circuit switching device 808 may be embodied as ap-channel metal oxide semiconductor (i.e., PMOS) transistor having agate connected to the output of the differential amplifier 804, a sourcecoupled to the source voltage 810, and a drain connected to the outputof the ramp generator circuit 801. In other words, the PMOS is beconnected in parallel with the capacitor to short circuit the capacitorwhen in an ON state (i.e., prevent the capacitor from charging). Inoperation, a trigger signal that transitions from a lower voltage to ahigher voltage at the input 802 of the current control circuit 800 maycause the PMOS to transition from an ON state to an OFF state to allowthe capacitor 812 of the ramp generator circuit 801 to begin charging.

The network of switching devices may further include a gate switchingdevice 818 to electrically couple the output of the ramp generatorcircuit 801 to the gate of the IGBT 824 (e.g., via the non-invertingamplifier 816). The gate switching device may be embodied as atransistor (e.g., n-channel metal oxide semiconductor transistor (NMOS))having its gate coupled to the input of the current control circuit(e.g., via the differential amplifier 804) so that when a trigger signalthat transitions from low to high is received at the input the gateswitching device 818, the IGBT 824 is coupled to the ramp generatorcircuit 801 (e.g., via the non-inverting amplifier 816).

The network of switching devices may further include a hard shut downswitch 822. The hard shut down switch may be embodied in various waysincluding (but not limited to) a NMOS transistor 822 as shown in FIG. 8.The hard shut down transistor 822 may be coupled at a gate terminal tothe input of the current control circuit 800 via an inverter 820 and viathe differential amplifier 804. The NMOS transistor 822 maybe coupled atits drain to the gate of the IGBT 824 and coupled at its source to aground voltage. In other words, the hard shut down switch 822 may pullthe gate-emitter voltage (V_(GE)) of the IGBT to the ground voltage whenturned to an ON state. The pull-down of V_(GE) turns the IGBT to an OFFstate and stops (e.g. abruptly stops) current from flowing in the coil826.

In operation, a trigger signal at the input 802 of the current controlcircuit 800 that transitions from a lower voltage to a higher voltageplaces the hard shut down transistor 822 in an OFF state, effectivelydecoupling it from the circuit and allowing the coil to be charged(i.e., through the IGBT that is turned ON by the gate switching device818). If a subsequent trigger signal is received a the input 802 thattransitions from the higher voltage back to the lower voltage, then thehard shut down transistor is turned ON, thereby turning OFF the IGBT andshutting down the coil current according to a hard shut down profile asdescribed previously. If the subsequent trigger is not received, thenthe hard shut down transistor remains off and the coil current iscontrolled by the ramp generator circuit 801 to shut down gradually(i.e., softly) after an over-dwell period.

The principles and techniques described can be applied as a method forcontrolling a current in a coil. The method 900 begins by receiving 920a charge coil trigger signal. This signal can be generated by a coilcontroller circuit or may be received from another circuit or system.For example, in a vehicle the charge coil trigger signal may be receivedfrom an engine control unit (ECU). The charge coil trigger signal may beembodied as a change in a voltage or a current. The change may beembodied in various ways including (but not limited to) a change inamplitude. For example, the charge coil trigger signal may be embodiedas a voltage transition from a lower voltage (e.g., a logical low level)to a higher voltage (e.g., a logical high level).

After receiving 920 the charge coil trigger signal, a soft-start rampsignal is generated 930. The soft-start ramp signal may be a voltagesignal that gradually decreases from a starting value (e.g., VSOURCE) toan ending value (e.g., a ground voltage) over a period of time (e.g., anover-dwell period plus a soft shut down period). For generating 930 thesoft-start ramp signal, the method may include coupling 931 couplingcapacitor to a source voltage (V_(SOURCE)), and charging 932 thecapacitor using an output current (I_(VCCS)) from a VCCS. The soft-startramp signal is related to the voltage across the capacitor (V_(CAP)) asit charges. To generate a soft-start ramp signal that has a shape thatgradually decreases the output current (I_(VCCS)) is controlled 932 bythe voltage across the capacitor (V_(CAP)).

The control of the output current (I_(VCCS)) based on the voltage of thecapacitor (V_(CAP)) depends on the operating characteristics (e.g., theI-V profile) of the VCCS. In some embodiments operating characteristics,such as the slope of the output current IVCCS versus the capacitorvoltage (V_(CAP)), or a minimum output current (I_(START)) or a maximumoutput current (I_(CHG)), may be set and/or adjusted 910 to match adesired over-dwell period (ODP) 901 or a desired soft shut down (SSD)period 902. The setting and/or adjusting operation may control the ODPand the SSD period independently and may occur once, periodically, or asneeded. For example, an ECU may determine a new ODP (e.g., to match anengine condition) and adjust a characteristic (e.g., VBIAS) of the VCCSso that the generated soft-start ramp signal results in the new ODP.

After the soft-start ramp signal is generated 930, it is used to control940 an operating point of the IGBT (e.g., according to thecharacteristic curve of the IGBT). The operating point of the IGBTdetermines the current through the IGBT and because the IGBT is inseries with a coil, the current through the coil. In some embodiments,the current through the coil may be limited (i.e., clamped) to a maximumvalue (e.g., due to the coil resistance) that is lower than theoperating point of the IGBT could otherwise support.

What results from the soft-start ramp signal controlling the currentthrough the IGBT, is a coil current with a profile (i.e., current vs.time) that rises quickly to a maximum value, at which it remains untilthe end of the ODP. After that time, the coil current gradually (e.g.,linearly) decreases until it reaches zero, which occurs at theconclusion of a soft-shut down period. This current profile starts whenthe first trigger signal is received so that if no subsequent triggersignal is received, the coil can be safely discharged.

In some embodiments, if a subsequent trigger signal (e.g., dischargecoil trigger signal) is received 950 (e.g., before the conclusion of theODP) then the control of the coil current by the soft-start ramp isconcluded and the coil current is immediately shut down 960. Forexample, in normal operation of an ignition system, the control of thecoil current by the soft-start ramp signal has no affect because thedischarge coil trigger is received before the ODP has elapsed and theimmediate shut down of the coil current (i.e., hard shut down (HSD))results in a spark (e.g., at a time corresponding the to discharge coiltrigger signal).

In the specification and/or figures, typical embodiments have beendisclosed. The present disclosure is not limited to such exemplaryembodiments. The use of the term “and/or” includes any and allcombinations of one or more of the associated listed items. The figuresare schematic representations and so are not necessarily drawn to scale.Unless otherwise noted, specific terms have been used in a generic anddescriptive sense and not for purposes of limitation.

Methods and materials similar or equivalent to those described hereincan be used in the practice or testing of the present disclosure. Asused in the specification, and in the appended claims, the singularforms “a,” “an,” “the” include plural referents unless the contextclearly dictates otherwise. The term “comprising” and variations thereofas used herein is used synonymously with the term “including” andvariations thereof and are open, non-limiting terms.

It will be understood that, in the foregoing description, when anelement, such as a component is referred to as connected to,electrically connected to, coupled to, or electrically coupled toanother element, it may be directly connected or coupled to the otherelement, or one or more intervening elements may be present. Incontrast, when an element is referred to as being directly connected toor directly coupled to another element there are no intervening elementsor layers present. Although the terms directly connected to or directlycoupled to may not be used throughout the detailed description, elementsthat are shown as being directly on, directly connected or directlycoupled can be referred to as such. The claims of the application, ifany, may be amended to recite exemplary relationships described in thespecification or shown in the figures.

Some described elements may be implemented using various semiconductorprocessing and/or packaging techniques. Some implementations may beimplemented using various types of semiconductor processing techniquesassociated with semiconductor substrates including, but not limited to,for example, Silicon (Si), Gallium Arsenide (GaAs), Gallium Nitride(GaN), Silicon Carbide (SiC) and/or so forth.

While certain features of the described implementations have beenillustrated as described herein, many modifications, substitutions,changes and equivalents will now occur to those skilled in the art. Itis, therefore, to be understood that the appended claims are intended tocover all such modifications and changes as fall within the scope of theimplementations. It should be understood that they have been presentedby way of example only, not limitation, and various changes in form anddetails may be made. Any portion of the apparatus and/or methodsdescribed herein may be combined in any combination, except mutuallyexclusive combinations. The implementations described herein can includevarious combinations and/or sub-combinations of the functions,components and/or features of the different implementations described.

1. A circuit for controlling a current in a coil, the circuitcomprising: a capacitor connected between a source voltage (V_(S)) andan output node; a first switching device connected between the voltagesource and the output node such that when the first switching device isin an ON state the output node is coupled to the voltage source, andwhen the first switching device is in an OFF state the output node iscoupled to the voltage source through the capacitor; and avoltage-controlled current-source (VCCS) connected between the outputnode and a ground, the VCCS outputting a current that charges thecapacitor such that when the first switching device is moved from the ONstate to the OFF state, a voltage is generated at the output node thatdecreases over a period from V_(S) to a ground voltage according to asoft-start-profile.
 2. The circuit for controlling a current in a coilaccording to claim 1, further comprising an insulated gate bipolartransistor (IGBT) having a collector coupled to the coil, an emittercoupled to ground, and a gate coupled to the output node such that thevoltage generated at the output node reduces the gate-emitter voltage(V_(GE)) of the IGBT to reduce a current through the IGBT and the coil.3. The circuit for controlling a current in a coil according to claim 2,further comprising a non-inverting amplifier coupled between the gatecoupled and the output node.
 4. The circuit for controlling a current ina coil according to claim 1, wherein the current output by the VCCSbetween a minimum current (I_(START)) and a maximum current (I_(CHG))corresponds to a voltage (V_(CAP)) across the capacitor as the capacitoris charged.
 5. The circuit for controlling a current in a coil accordingto claim 4, wherein for the current output by the VCCS equals I_(START)when V_(CAP) equals zero volts.
 6. The circuit for controlling a currentin a coil according to claim 4, wherein the current output by the VCCSbetween a minimum current (I_(START)) and a maximum current (I_(CHG))corresponds to a voltage (V_(CAP)) is linearly related to V_(CAP) by aslope (dI/dV).
 7. The circuit for controlling a current in a coilaccording to claim 1, wherein the voltage generated at the output nodedrives a gate of a switching device to control the current in the coil.8. The circuit for controlling a current in a coil according to claim 7,wherein the soft-start profile of the voltage generated at the outputnode driving the gate of the second switching device configures thesecond switching device to reduce the current in the coil to zero over asoft shut-down period that begins after an over-dwell period.
 9. Thecircuit for controlling a current in a coil according to claim 8,wherein the soft shut-down period and the over-dwell period are eachcontrolled by the soft-start profile.
 10. The circuit for controlling acurrent in a coil according to claim 1, wherein the soft-start profileis determined by characteristics of the voltage controlled currentsource.
 11. The circuit for controlling a current in a coil accordinggot claim 10, wherein the characteristics include a minimum current(I_(START)), a maximum current (I_(CHG)), and a slope (dI/dV) of anoutput current versus an input voltage.
 12. A method for controlling acurrent in a coil, the method comprising: receiving a charge coiltrigger signal generating a soft-start ramp signal that has a voltageprofile that decreases with time; and applying the soft-start rampsignal to a gate of a switching device to gradually shut down thecurrent in the coil over a soft shut-down period after an over-dwellperiod, wherein the soft shut-down period and the over-dwell periodcorrespond to the voltage profile of the soft-start ramp signal.
 13. Themethod according to claim 12, wherein the generating a soft-start rampsignal includes: coupling a first side of the capacitor to a sourcevoltage; controlling a current charging the capacitor using a voltagecontrolled current source (VCCS) that is connected between a second sideof the capacitor and a ground voltage; and outputting the soft-startramp signal as the voltage at the second side of the capacitor.
 14. Themethod according to claim 13, wherein a voltage across the capacitor isapplied to the input of the VCCS to control current charging thecapacitor.
 15. The method according to claim 13, wherein the voltageprofile that decreases with time is controlled by operatingcharacteristics of the VCCS.
 16. The method according to claim 15,wherein the operating characteristics of the VCCS include a minimumcurrent, a maximum current, and a slope (dI/dV).
 17. The methodaccording to claim 15, further comprising: adjusting the operatingcharacteristics of the VCCS to provide a particular over-dwell period ora particular soft shut-down period.
 18. The method according to claim12, wherein the switching device is an insulated gate bipolar transistor(IGBT).
 19. A soft-start ramp generator, comprising: a voltage sourceterminal; a ground terminal; an output node; a capacitor coupled betweenthe voltage source terminal and the output node; and a voltagecontrolled current source (VCCS) coupled between the output node and theground terminal, the VCCS configured to control the current through thecapacitor based on the voltage across the capacitor so that a voltage atthe output node decreases from a voltage at the voltage source terminalto a voltage at the ground terminal according to a soft-start profile.20. The soft-start ramp generator according to claim 19, wherein voltageat the output node of the soft-start ramp generator is coupled to aswitching device to control a current in a coil to gradually decreaseover a soft shut-down period after an over-dwell period.